net80211: add one-vap version of ieee80211_iterate_nodes()

• Add a counter into 'struct ieee80211_node_table' to save current number

of allocated nodes.
(allows to remove array overflow checking in ieee80211_iterate_nodes()).

• Add ieee80211_iterate_nodes_vap() function; unlike non-vap version,

it iterates on nodes for specified vap only.

In addition to the above:

• Remove ieee80211_iterate_nt(); it is not used by drivers / net80211

outside ieee80211_iterate_nodes() function + cannot be separated due
to structural changes in code.

Since size of 'struct ieee80211_node_table' (part of ieee80211com,
which is a part of driver's softc) is changed all wireless drivers /
kernel need to be recompiled.

Tested with wpi(4), STA mode.

Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D7996